Temperature-pressure activated purge gas flow system for flares

ABSTRACT

A system for supplying purge gas to a flare system depending upon the temperature and the pressure in the flare, to control the flow of purge gas through the flare during the period when flaring of gas is discontinued. The rate of flow of purge gas is determined by temperature measuring means. However, the control of flow depends upon the pressure in the flare as well as the temperature so that when gas is being flared and the temperature is high and the pressure is above normal the purge gas is cut off, whereas when the temperature is high and the pressure is normal the purge gas is flowed.

United States Patent [1 1 Reed et a1.

[ TEMPERATURE-PRESSURE ACTIVATED PURGE GAS FLOW SYSTEM FOR FLARES [75]Inventors: Robert D. Reed; John S. Zink;

Robert E. Schwartz, all of Tulsa, Okla.

[52] US. Cl. 431/202; 23/277 C [51] Int. Cl. F23d 13/20 [58] Field ofSearch 431/202, 5, 29, 89;

[56] References Cited UNITED STATES PATENTS 2,779,399 1/1957 Zink et al.431/202 Wilkinson 431/202 Evans et al 23/277 C Primary ExaminerEdward G.Favors 5 7 ABSTRACT A system for supplying purge gas to a flare systemdepending upon the temperature and the pressure in the flare, to controlthe flow of purge gas through the flare during the period when flaringof gas is discontinued. The rate of flow of purge gas is determined bytemperature measuring means. However, the control of flow depends uponthe pressure in the flare as well as the temperature so that when gas isbeing flared and the temperature is high and the pressure is abovenormal the purge gas is cut off, whereas when the temperature is highand the pressure is normal the purge gas is flowed.

9 Claims, 3 Drawing Figures PATENTEDAUBZBiQYS F/G. Z

TEMPERATURE-PRESSURE ACTIVATED PURGE GAS FLOW SYSTEM FOR FLARES I CROSSREFERENCE TO RELATED PATENTS This invention is related to U.S. Pat. No.3,741,713

issued June 26, 1973, entitled: PURGE GAS ADMIS- SION CONTROL FOR FLARESYSTEM.

BACKGROUND OF THE INVENTION In chemical and petroleum refining systemsit is necessary to maintain a flare stack through which wastecombustible gases can be released and burned in such a way as safely tobe disposed of, with a minimum of pollution. Often very large quantitiesof flare gas become available in emergency situations, which gases are,at times, at a temperature in excess of 250 to 300 F. A constantlyburning pilot flame is provided so that any gas introduced into theflare stack will be ignited without fail at exit.

When the flaring of gasis stopped at the conclusion of the emergency,the stack, and the gases in the stack, which have been at an elevatedtemperature, begin to cool-to ambient temperature. Asa result, thepressure of the gas in the stack reduces proportionately to the absolutetemperature of the gas. This reduction in pressure permits atmosphericair to be drawn into the top of the flare stack since flow has stopped.This creates a dangerous situation. When the next flaring of gas isrequired, because of the combustible nature of the flared gas, thepresence'of the air can provide an explosive mixture which is easilyignited because of the constantly burning pilot.

In order to keep the system air free at all times, it is common to admitto the system a continuous flow of purge, or sweep gases, to maintainconstant slow movement of gases through the system and out to the exitpoint. Two factors govern the purge or sweep gas movement. One factor isdue to the passage of wind blowing across the open discharge end of thestack. .To counter this effect it is common to use what is called amolecular seal in which there are two flow reversals. Such seals aredescribed in U.S. Pat. Nos. 3,289,729 and 3,055,417. This allows entryof outside air only to the structure downstream of the molecular sealwhich is'placed generally immediately below the point at which ignitionoccurs. For large flare stacks this is an expensive addition to thestack and does notprovidea complete solution to the problem.

' The second factor is temperature change within the system, duetoeither meteorological conditions, or due to the flaring of wastegases-at significant temperature level. For example, in a flare stack 30inches in diameter and 500 feet tall the system volume is approximately2400 cubic feet. Assuming such a volume is filled with flare gases at250 F. during a flaring period and wheri the discharge of hot gases isstopped, the systern will cool to ambient temperature in about minutes IThe volume of gases within the flare stack at atmospheric pressure woulddecrease proportionately to the absolute temperature to approximately1700 cubic feet. Thus, air will be drawn into the stack in the amount of2400 minus 1700 or approximately 700 cubic feet. This would cause an airpenetration down the stack of approximately 150 feet. Since this columnof air wouldv travel for approximately 15 minutes it cor- 30 inch stackwould require about 700 cubic feet of purge gas, or to provide a marginof safety, approximately 800 cubic feet each 15 minutes. The hourlyvolume would be about 3200 SCFH.

When the system is at ambient temperature there is no longer need forthis large flow of purge gas at 0.15 feet per second. A nominal flowvelocity of about 0.03 feet per second is adequate to insure that air iskept out of the stack at all'times.

DESCRIPTION OF THE PRIOR ART The prior art, as represented by the patentU.S. No. 3,741,713 is inadequate for several reasons:

1. Wastefulness of purge gases, which generally are hydrocarbon gases,or fuel gases. This waste is contrary to present day fuel-heat energyconservation requirements.

2. The prior art systems do not respond to small temperature changes inflare contained gases, which can be shown to be dangerous. I

3. The prior art systems do not prevent the entry or use of purge gaseswhen there is flow within the flare system and purge gases are notrequiredfor flare safety.

4. In the case of flare systems which operate with liquid seals at theflare base, constant purge or sweep gas flow is not required to furtherbenefit fuel-energy conservation.

SUMMARY OF THE INVENTION The terms purge or sweep gas are usedinterchangeably. Such gas can be any substance which is in gaseous phaseat any temperature to which it may be subjected in this system. Commonlyused purge gases are methane, ethane, propane, and nitrogen, but othergases such as argon can be used.

The purpose of the use of purge gases in flare systems is to avoidallowing a static, or reversed flow, in the entire flare system and toalways maintain some small flow toward the discharge point of the flaresystem. The flare system does not become dangerous until such time asair or oxygen is present within the system; If the system flow isallowed to reverse,'air is drawn into the system and there is immediatehazard of explosion or detonation, whenever normal forward flow isreestablished andthe air-gas mixture meets the pilot flame.

Practice in the process industries where flares are used, has been toadmit purge gas to the flare system to cause movement within the systemtoward the flare at commonly accepted flow velocity of 0.05 feet persecond,as a minimum. Thisflow may seem small, but for such movement in a24 inch flare system the purge gas required is 540 SCFH. For a 36 inchsystem the gas required is 1,237 SCFH, and for a 48 inch system the gasrequired is 2,215 SCFH. All of these are common system sizes. In somecases this volume of purge gas can be saved, and in the case of methanethe saving is a startling $2,365 per year for a 24 inch system or $9,700per year for a 48 inch system. Some process plants may have as many asfive flares.

The purge rate of 0.05 feet per second is considered satisfactory for adrop in system temperature of 20F in 15 minutes (such as from F. to60F.) But, if the temperature drop should be 40 F. in 15 minutes it isnecessary to increase purge gas volume to maintain the same condition offlare safety because the gas volume is proportional to absolutetemperature at the same 3 pressure condition. Ambient conditions ofsunshine, or chilling rain orwind action, plus a drop in atmospherictemperature, combined with above ambient stack gas temperature, canproduce minute temperature drops as great 'as 90Ffur'ider summerconditions: At

90F. drop in l5 minutes'the'purge gas movement must be 0.083 feet persecondfo'r safety maintenance. I The data just shown are jilstifiedbycalculation of'the decrease in flare gas volume w hen ther e'is a dropin temperature of the gas contained in. the flare system when there isno flow. For example, in a drop of temperature fr'dmsom, to 60F.'thevolume v within the flare system drops to O I963 V. With a dro p from150F. to 60F. the volume'\'/ is reduced to'0. 852 V at the same pressurelnthe case of dropintemperature from 80F. to 60F: this requiresa'volum'e of purge gas of 0.037 V or a flow of purge gas at the rate of0.050 feet per' second. For thecase'of drop from 150F. to 60F. a volumeof purg'e'gas of 0.148 V is required or a flow rate ofpurge gas of 0.083feet per second lt'is thus to'be seen that in"'n'orma'l meteorologicalchanges, safety requires that purge gas volume be modulated to'suit thetemperatur'e'condition of the gas contentof the flaresystem when reliefventing of waste gases is not present. This is according to normalweather'c onditions; but in relief of hot gases from process operationsthe gas temperature can be as high'as 500F. or more and purge gasdemand-is .greatly in creased to 0.619 feet per' second to further addto the rieed'for gaslmodulation according to t'he'system ternfper'ature.

j The htior ar't such as 'illus trated by'Patent 3,741,713 is dlirectedto the higher'temperature condition but it makes no provisionfor normalweather changes when, as shown, there is need for added purge rate. Theprior also makes'no provision for purge rate modulation according to theflare system g astem'perature' for purge gas conservation. i

& SUMMARYOFTHE I NVENT IONi obje'ct of this invention to provide a purgesystem which'conserve's fuel heat-energy by controlling the rate of pu'rg'egas flow'in accordance with the temperature of thegasin the flaresyste r'nfand the pressure in the flare system. When the temperature isabove normal and the pressure is normal purge gas 'is 'supplied in accordance with thetemperatureJWhen the temperature is high and the pessure is also high flow of purge gas is not allowed to occur."

BRIEF DESCRIPTION OFiTHE DRAWI GS the FTC. l represents a sitiiation'inwhich a water s'eal'is provided in the flow system of the flare.

FIGS. 2 aha 3 illustrate the situation where there is no water seal inthe flare systein, but the flare gas is controlled specifically'by thetemperature in the flare system and the pressure in the flare system.

,. DESCRlPTIQNOFTlHE PR FEaREo EMBODIMENT Before eitplaining'tliepresent invention'in detail; it is applicatiorito'the details ofconstruction and arrange ment of parts illustrated in the accompanyingdrawings, since the invention is capable of other embodiments and ofbeing practiced and carried out in' various ways.

Also it is to be'understood that the phraseology, or terminology,employed herein is for the purpose of description and not of limitation.

Referring now to the drawings and in particular to FIG. l there is showna flare system indicated generally by the numeral 10; a purge gas systemindicated generally by the numerallZ, and a pressure control systemindicated generally by the numeral 14. The system of FIG. 1 includes aspart of the flow system, a water seal in the base of the flare stackwherein a column of water 28 has immersed in it, the flare gas conduit24 which has a downwardly depending portion 26 immersed in the water toadepth H. The depth H is sufficient so that there'is substantially nodanger that the water level will be drawn down to the point where gas inthestack-can leak back into the conduit 24. I

' The base of the stack 16 rests on the grade surface 46 and comprises aplurality. of sections 18 terminating at the top opening 22. There is apilot fixture providing a-constant flame for igniting the wastecombustible gases that will flow in accordance with the arrow 13 throughthe conduit 24-26 and bubble up through the water '28 and flow upwardlythrough the flare stack 18 to the top opening 22 where it will beignited and will burn. z w

When the flow of gas stops, the water seal at 28 prev'ents any of thegases in the stack 18 from re-entering the pipe 24. However, to furtherprevent this flow backward there is a pressure connection 32 from theconduit 24 to a pressuresensitive switch 34 which is supplied withelectrical power over leads 36. When the pressure in the conduit 24 islow, power is applied through two leads 38 to the electricallycontrolled valve 40 which permits purge gas from input pipe 42 to passth'rough'the valve 40and through line 44 into the conduit 24 to maintainthe pressure in the .conduit at a preselected pressure level. With thispressure at the se- "lected level there is no tendency for gas to bewithdrawn from the stack back into the conduit 26.

' In accordance with the teachings of this invention,

measurements are made of the gas temperature in,the

flare stack and the pressure of the gas in the flare stack 'toso controlthe flow of purge gas as to prevent any entry of air into thestackthrough the opening 22, and to provide thiscontrotwith a minimum totalflow of purge gas, for the purpose of fuel and cost saving.

' A temperature sensor 48 which can be of a thermistor, thermocouple,capillary, or other type in accordance with the teachings of US. Pat.No. 3,741,743 is inserted into the-flare stack 16 18. With a suitablecontrol means 50, as well known in the .art, a signal is sent to athermal controlller T indicated by numeral 52.

:Power supplied by leads 54 to the thermal controller is 'ture' in thestack is above normal, purge gas could be permitted to flow through thevalve 58 into the stack. Although the prior art indicates that thisthermal control issatisfactory, it is in fact not fully satisfactoryand, therefore, it becomes desirable to control the gas further bymeans, of a pressure switch P indicated by numeral 56. This is connectedby conduit 64 to the flare stack 18 and the pressure switch 56 isinterposed between the leads 53 and 55. When the temperature at 48 isabove normal the temperature control device T 52 provides a signal tothe valve 58 to cause purge gas to flow through lead 62. However, whenthe flare gas is flowing, there is no need for purge gas because thereis flow upwardly in 18 and the pressure in 18 is greater than normal.Thus when the temperature indicated by thermal sensor 48, andthepressure on conduit 64 are both above normal, the pressure switch 56prevents the passage of signal from the temperature controller 52 to thevalve, so the valve remains closed. However, when the flow of flare gasstops, the pressure will reduce ,to normal, while the temperature stillremains high. Therefore, under these conditions ;it is necessary to flowpurge gas, and the pressure switch 56 then closes permitting the signalfrom thethermal' control 52 to control the valve 58 and cause purge gasto flow into the flare stack. 'Asthe purgegas flows and as the gas inthe flare stack cools the temperature indicated by sen sor 48 eventuallyreaches normal value and the thermal control 52 causes the valve 58 to.close and cut the flow of purge gas to the stack. y

In FIG. 1 the control system indicated by numeral 12 employing thermaland pressure switches provides a more controlled flow of purge gas inaccordance with the temperature and pressure in the stack than does theprior art. FIG. I shows also an additional feature which has been usedin prior art systems namely of the water seal at 28. However, in FIG. 1there is a further control indicated by numeral 14 which, as needed,delivers a flow of purge gas in to the conduit 24 to maintain pres sure.This involves the pressure switch 34 and control valve 40.

When flared gas flow in conduit 24 ceases, the immediate restingpressure in 24 is H inches water-column and if the gas inside 24 is atambient temperature and" if there is no leakage from 24, the restingpressure does not change. But if the gases inside 24 are at elevatedtemperature, there is reduction in both volume and pressure due tocooling or pressure in 24 is reduced by leakage, correction isimmediately needed.

Pressure in 24 should always be above atmospheric pressure to avoidleakage-entry of air to 24; also to avoid withdrawing water 28 from thebase of 16 which can occur if initial gas temperature is high enough asgas flow ceases. Normal pressure in 24 is greater than atmospheric.

The system indicated by 14 incorporates means to admit purge gas fromsubstantial pressure to 24 when the pressure internal of 24 falls tojust above atmospheric pressure. Pressure switch P senses the internalpressure of 24 through 32. As the pressure internal of 24 falls for anyreason to near atmospheric pressure, the switch P 34) closes. Power from36 then is applied through 38 to a gas valve 40 which then opens toallow passage of purge gas from 42 to 44 and thence to 24 for immediaterestoration of pressure within 24.

The control valve 58 is preferably one in which the rate of flow ofpurge gas through the valve from pipe 60 to pipe 62 is variablycontrolled in accordance with the temperature difference between thesensor temperature and the preselected ambient temperature, so that asthe temperature difference increases the rate of flow of purge gasincreases proportionately. The type of to line 44. However, when flowceases in 24 and pressure inside 24 falls to just above atmospheric forsome reason, controls 14 immediately admit purge gas from 42 through 40and 44 to 24 for immediate restoration of pressure in 24. Control of therate of flow of purge gas through valve 40 is not precisely controlledas is the fb w through valve 58.

In FIGS. 2 and 3 there is shown a modification of the system of FIG. 1in which the water trap comprising the water .column 28 and invertedconduit 26, is 'not presem and the entire control of purge gas is byasystem similar to that indicated generally by numeral 12 in FIG. 1.There is in FIG. 2 for example, the same sensor 48, sensor control 50,lead 51 to a temperature controller 52 supplied withpower on leads 54and connected to leads 53, and 55 to a flow valve 76 connected betweenthe line 78 carrying the purge gas and line 80 connected to the conduit72, through which the waste flare gas passes into the flare stack 70. Inaddition, there is a pressure lead 74 connected from the conduit 72 to apressure switch 56 interposed between the temperature controller 52 andthe valve 76.

As in FIG. l'the temperature sensor 48 controls the valve 76 as afunction of the temperature measured. However, the control signal fromthe thermal controller 52 is controlled further by the pressure switch56 responsive to the pressure in the conduit 72 which is indicated onlead 74.

Here again, when the temperature of the flare gases moving into thestack is high, and the pressure is low, the pressure switch 56 is closedand transmits the signal from the thermal controller 52 to the valve 76permitting a controlled flow of purge gas through line 80 into theconduit 72 and to the stack. On the other hand, if the pressure in theconduit 72 is high and the temperature at 48 is also high, indicatingthat there is a flow or flare gas at that time, there is no need forpurge gas so thatthe pressure switch 56 will open and prevent the signalfrom the thermal controller reaching the valve 76.

As in FIG. 1 there are two main differences between this system and theconventional prior art system, namely that the control of purge gas ismade dependent upon both the gas temperature in the flare stack and thepressure in the flare stack, whereas in the prior art the control ofpurge gas was made solely in the basis of temperature. Furthermore,there is the improvement of having a variable flow valve to control thepurge gas, the rate of flow being a selected function of the temperaturedifference between the existing stack gas temperature and normaltemperature depending upon the season and the weather and other localfactors.

In FIG. 3 is shown a system similar to that of FIG. 2 except that ituses a type of thermal sensor which is a liquid-filled capillary type ofsensing device well known in the art. The type of thermal control 88would be different from the control 52 inasmuch as the thermal sensor 86is different from that of 48. However, the commercial devices which areavailable on the market provide the proper control between temperatureand switch opening and closing, so that with the proper control deviceany one of the conventional types of thermal sensors can be used, as iswell known in the art.

While the control system is shown as a series of separate elements, suchas sensor 48, sensor control 50, lead 57, temperature controller 52,pressure switch 56 and valve 76, these elements could be combined in anydesired way to provide the equivalent, overall system. Irrespective ofthe individual elements and their names, etc., the novelty of thissystem resides in the dual control of purge gas based on the temperatureand the pressure in the flare system. It also includes proportionalcontrol of the purge gas based on temperature difference.

In FIG. 1 where there is a water-seal formed by immersion of 26 in 28 todepth H, immersion is critical to avoid reversed gas flow from theinterior of 16 above 28. To provide evidence of the existance of 28 alevel indicating device 30 is provided. Visual check of level at 30 is agesture toward safety but in the absence of visual check, an alarm canbe sounded when the level of 28 falls because of leakage, drainage orother cause. Any typical alarm device can be used. The element 30 canalso be adapted, by means commercially available, for addition of waterto 28 if it is needed for any reason and as it is needed normally.

While the invention has been described with a certain degree ofparticularity, it is manifest that many changes may be made in thedetails of construction and the arrangement of components. It isunderstood that the invention is not to be limited to the specificembodiments set forth herein by way of exemplifying the invention, butthe invention is to be limited only by the scope of the attached claimor claims including the full range of equivalency to which each elementor step thereof is entitled.

What is claimed:

1. The temperature-pressure activiated purge gas flow system for wastegas flares comprising;

a. a flare gas system including flare stack conduit means to introduceflare gas into said stack, pilot ignition means and means to introducepurge gas into said system;

b. means to measure the temperature and the pressure in said flare gassystem and to determine the temperature difference between saidtemperature and a selected ambient temperature; and

0. means responsive to said temperature difference and to said pressureto control the flow of purge gas into said flare gas system.

2. The system as in claim 1 including control valve means responsive tosaid means to control the flow of purge gas into said system.

3. The system as in claim 2 in which, when said temperature differenceis greater than a selected value, said control valve tends to open.

4. The system as in claim 3 in which when said pressure is higher than aselected valve, said control valve is prevented from opening.

5. The system as in claim 3 in which, when said pressure is lower than aselected valve, said control valve is permitted to open.

6. The system as in claim 2 in which said control valve is controlledproportionately by said temperature difference.

7. The system as in claim 1 including water trap means between saidconduit means to introduce flare gas into said stack, and said flarestack.

8. The system as in claim 7 including in addition means to maintain aselected minimum gas pressure in said conduit means.

9. The system as in claim 7 including means for maintenance of saidwater trap.

1. The temperature-pressure activiated purge gas flow system for wastegas flares comprising; a. a flare gas system including flare stackconduit means to introduce flare gas into said stack, piLot ignitionmeans and means to introduce purge gas into said system; b. means tomeasure the temperature and the pressure in said flare gas system and todetermine the temperature difference between said temperature and aselected ambient temperature; and c. means responsive to saidtemperature difference and to said pressure to control the flow of purgegas into said flare gas system.
 2. The system as in claim 1 includingcontrol valve means responsive to said means to control the flow ofpurge gas into said system.
 3. The system as in claim 2 in which, whensaid temperature difference is greater than a selected value, saidcontrol valve tends to open.
 4. The system as in claim 3 in which whensaid pressure is higher than a selected valve, said control valve isprevented from opening.
 5. The system as in claim 3 in which, when saidpressure is lower than a selected valve, said control valve is permittedto open.
 6. The system as in claim 2 in which said control valve iscontrolled proportionately by said temperature difference.
 7. The systemas in claim 1 including water trap means between said conduit means tointroduce flare gas into said stack, and said flare stack.
 8. The systemas in claim 7 including in addition means to maintain a selected minimumgas pressure in said conduit means.
 9. The system as in claim 7including means for maintenance of said water trap.